Fireproof is a very useful perk that negates fire and slowly take damage from lava. This can help in situations where a meteor is impacting the land and burning it, or if the land under you is destroyed and fall in lava. You're also immune to Fireball and Spicy Turkey.
If you do end up falling into lava, you will take 20 damage slowly.
Perk Tips Edit
If you have the "Second Perk Slot". Fireproof goes extremely well with Double Jump as it's one of the best combos to use on intensity mode. So you don't die as often.
Fireproof also once used to be called Lavaproof, and had a different function where you were completely immune to lava, however still took damage from hot parts.
Not only was this incredibly overpowered, it also made no sense to be able to survive lava, just to die from a burning brick. This led to Polyhex changing the name and function of this perk.